Grinding of magnetic ores



May 5, 1970 D, WESTON 3,510,058

GRINDING OF MAGNETIC ORES Filed Nov. 27, 1967 2 Sheets-Sheet 1 /2 W TMm/6s May 5, 1970 Filed Nov. 27,

D. WESTON GRINDING OF MAGNETIC ORES 2 Sheets-Sheet 2 United States Patent O 3,510,068 GRINDING F MAGNETIC ORES David Weston, Suite 500, 129 Adelaide St. West, Toronto 1, Ontario, Canada Filed Nov. 27, 1967, Ser. No. 685,810 Claims priority, application Canada, Dec. 12, 1966, 977,798, Patent 814,997 Int. Cl. B02c 23/00 U.S. Cl. 241-25 3 Claims ABSTRACT OF THE DISCLOSURE An improvement in the grinding of magnetic ores wherein the magnetic material in the ore is magnetically flocculated before being fed to the mill.

BACKGROUND OF THE INVENTION This invention relates to the grinding of magnetic ores and ores in which at least one of the mineral constituents is readily magnetizable such for instance as magnetite, and other iron ores and various ores of nickel, tin, zinc and pyhrrotite.

In conventional grinding circuits for grinding magnetic iron ores the product from the grinding units is usually passed to a magnetic cobber which produces a discardable tailing and a magnetic concentrate which is then passed through a demagnetizing coil before being pumped through a cyclone-type classifier. The classifier undersize represents a final concentrate and the classifier oversize is returned to the milling unit for further grinding.

SUMMARY OF THE INVENTION I have found that in milling circuits of this character a surprising increase in grinding capacity can be obtained if just prior to its reentry into the mill the recirculated material is subjected to the influence of a magnetizing coil having a field of sufficient strength to magnetically fiocculate the bulk of the magnetically susceptible particles of mineral. Similar benefits are obtained by magnetizing the total feed material in the sarne way Where the fresh feed contains any substantial quantity of magnetically susceptible fines. Furthermore if the milling circuit is a dry circuit, similar benefits can be obtained by magnetizing the recirculating material or the total or part of the feed just before the latter is fed to the comminution unit.

The increases in grinding capacity brought about according to the invention are of the order of to 15 percent and appear to arise as a result of the fiocculated magnetized material being attracted on entry to the mill to the surfaces of the balls and mill shell where they are held momentarily at least in a position where the grinding action of the mill is most effectively applied to them. The particles themselves lose their magnetism as grinding work is done on them and accordingly it must be deduced that the advantage of the invention derives almost entirely from the favourable grinding conditions which are achieved immediately after the material enters the mill.

Where the mineral concerned is magnetite the magnetizing coil may be of relatively low-field intensity of from about 400-700` gauss and may for instance in the case of a wet circuit take the form of a magnetizing coil surrounding the underow return line adjacent its point of discharge into the feed chute of the mill. Where other types of mineral are concerned, such for instance as hematite or nickel bearing ores, a field intensity of up to about 20,000 gauss or higher may be employed.

In carrying out the invention it is important that the magnetically flocculated material be fed directly to the mill as soon as possible after it has been occulated and without passing through any pumps or being subjected 3,510,068 Patented May 5, 1970 to any vigorous mechanical agitation, since pumping or vigorous mechanical agitation have a deflocculating action which destroys the effectiveness of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The invention and its operation will be more fully understood from the following description of typical grinding circuits employing the same which are described in conjunction with the accompanying drawings wherein:

FIG. l illustrates a simple wet grinding circuit typical of those used in the comminution of magnetic ores.

FIG. 2 illustrated a more complex grinding circuit employing dry comminution in a primary milling circuit followed by a wet milling regrind circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. l the circuit components consist of a Wet milling unit 10 which may be any conventional type of Wet mill such for instance as a ball mill or an autogenous mill; a magnetic cobber 11 which may be of conventional design and may, instead of a single cobber as shown, consist of a number of cobbers arranged in series according to the practice in the plant concerned; a concentrate sump 12 arranged to receive concentrate from the cobber 11; a pump 13 arranged to pump concentrate from the sump 12; a demagnetizing coil 14 around the line between the sump 12 and the pump 13; a cyclone classifier 15 arranged to produce as overflow a final concentrate from the circuit and an underflow which is returned to the mill 10. According to the invention there is placed in the line returning the underflow to the mill the magnetizing coil 16 which is positioned adjacent the point of discharge of the underflow into the mill inlet 17.

The circuit just described is adapted to receive relatively coarse feed material containing no substantial percentage of fines and consequently the magnetizing coil 16 is arranged so as to flocculate only the classifier underfiow. However, if the feed to the circuit were to consist of material containing a sufhcient quantity of fines to enable a substantial amount of magnetic flocculation thereof, the magnetizing coil 16 would be so arranged as to magnetize both the underfiow from the classifier and the incoming fresh feed material.

In the circuit illustrated in FIG. 2 the primary grinding unit is a dry mill 20 which may typically be a large diameter short length mill of the type for instance described and claimed in Weston prior Pat. No. 2,704,636. The mill is fed by a circuit including a primary crusher 21 which delivers crushed material to stock pile 22 from which ma terial is fed by apron feeders 23 to the feed conveyor 24 which discharges material into the feed chute 25 of mill 20. The mill product is discharged in a current of air and passes through primary and secondary classifiers 26 which remove a coarse fraction therefrom. This coarse fraction is delivered to a series of dry magnetic cobbers 27 which produce a concentrate 28, a middlings 29 which are returned to the mill 25 and a tailing 30 which is then slurried and passed to Wet magnetic cobbers 31 which produce a final tailings 32 and a concentrate 33.

The portion of the mill product which is not collected by the primary and secondary classifiers is collected in cyclone collectors 34, the extreme fines being collected in a venturi scrubber 35. These products are slurried and passed to the wet magnetic cobbers 36 producing a tailings 37 and a concentrate 38. The concentrates 33, 28 and 38 are combined in a total concentrate 39 which is fed to a wet secondary grinding mill 40.

The product of wet mill 40 is fed to wet magnetic cobbers 41 which produces a discardable tailing 42 and a concentrate 43. The concentrate 43 is passed through a demagnetizing device 44 before being passed through cyclone classifier 45 Wherein the fines 46 are passed to a finisher magnetic circuit 47 and nally form a final concentrate 48. The classifier underow is recirculated to the mill 40.

According to the invention the middlings 29 are magnetically flocculated by the magnetizing coil 50 immediately before their passage into the feed chute 25 of the mill 20. The primary mill feed consisting essentially of coarse material which does not contain any substantial amount of fines is not magnetized since it is too coarse to exhibit any worthwhile degree of occulation. However, since all the feed to the wet mill 40 is relatively ne both the oversize 49 and the total concentrate 39 are subject, according to the invention, to ilocculation by the magnetizing coil S1 immediately before entering the feed chute S2 of the wet mill 40.

As an example of the benets obtained according to the invention a circuit was operated according to the schematic illustration in FIGURE 1 using as the mill 10 a twelve foot diameter combined wet crushing and grinding mill. The original ore contained approximately 39% iron as magnetite and the classier oversize return from the cyclone classifier 15 contained approximately 50% iron essentially as magnetite. The nal product of the cyclone overilow was 58 to 63% minus 325 mesh. The circuit was first operated without the magnetizing coil 16 and when balanced conditions were achieved the capacity of the mill was 11.8 long tons per hour. The magnetizing coil 16 was then energized with initially no apparent eiect upon mill capacity. As the energization of the magnetizing coil 16 was increased heavy flocculation of the classifier underflow was observed and concurrently with the observation of this phenomenon the capacity of the circuit began to increase. When balanced conditions had again been obtained with the circuit producing the same product and grind as before the capacity of the circuit had increased to 13.4 long tons per hour or approximately 13.5%.

What I claim as my invention is:

circuits for ores containing magnetic or magnetically susceptible minerals which comprises subjecting material fed to a mill to a magnetic field of sufficient intensity to magnetically occulate the bulk of said materials, the said magnetic iield being applied at a point prior to but suiciently close to the point of entry of said material into the mill that said minerals are in substantially ilocculated condition when they actually enter the mill.

2. A method as claimed in claim 1 wherein the ore fed to the mill contains a substantial proportion of magnetically susceptible fines and the whole of such feed is subjected to a magnetic iield of suicient intensity to magnetically occulate the bulk of said magnetically susceptible Iines.

3. A method as claimed in claim 1 wherein the material fed to the mill contains a coarse fraction and a magnetically susceptible iine fraction and only the said iine fraction is subjected to a magnetic eld of sufiicient intensity to magnetically flocculate the bulk of the particles of said fraction.

References Cited UNITED STATES PATENTS 1,857,879 5/1932 Schubardt 241-25 X 2,287,804- 6/ 1942 Johnson 241-25 X 2,612,262 9/1952 Symington et al. 209-39 X 3,022,956 2/ 1962 Haseman 241-29 X FOREIGN PATENTS 251,470 5/ 1964 Australia.

FRANK T. YOST, Primary Examiner U.S. C1. X.R. 209-5; 24l--29, 30y 

